1. Field of the Invention
The invention relates to a measuring device with at least one micro-electromechanical capacitive sensor, which has electrodes that move toward and away from each other for measurement of a mechanical deflection of a test mass, with a charge integrator comprising an operational amplifier that has at least one amplifier input connected to the sensor and at least one amplifier output that is fed back to the amplifier input via at least one integration capacitor.
2. Description of Related Art
Such a measuring device is disclosed in Green, J. et al. “Single-Chip Surface Micromachined Integrated Gyroscope with 50°/h Allan Deviation,” IEEE Journal of Solid State Circuits, vol. 37, no. 12 (2002), pp. 1860-1866. The measuring device is part of a gyroscope that comprises a test mass, which is deflected from a rest position against a spring force of a first spring toward a first axis, and against a spring force of a second spring toward a second axis at 90 degrees relative to the first axis. The gyroscope has an electromechanical oscillator, which comprises an excitation mechanism by means of which the test mass is set into a primary oscillation toward the first axis. The movement of the test mass toward the first axis is measured by means of a primary capacitive sensor. By means of a feedback loop, a rectangular voltage is generated as a function of the measuring signal thus received and said voltage is applied to a capacitive actuator driving the test mass. The phasing of the rectangular voltage is selected so that the test mass oscillates with the resonance frequency of the primary resonator formed by the first spring and the test mass.
A turning of the test mass about a rotation axis running perpendicular to the first axis and to the second axis generates a Coriolis force, which brings about a secondary oscillation of the test mass toward the second axis. The corresponding deflection is measured by means of a secondary capacitive sensor. Because the Coriolis deflection of the test mass is less than 2 Å, the measuring signal of the secondary sensor is amplified. To this end, the secondary sensor is connected to an amplifier input of an operational amplifier, of which the amplifier output is fed back to the amplifier input via an integration capacitor to form a charge integrator. Connected in parallel to the integration capacitor is a source-drain channel of a field effect transistor (FET), which forms a gigaohm resistor and counteracts a displacement of the DC voltage potential at the amplifier input. By means of the resistor, the operational amplifier is prevented from over-modulating due to an existing input offset voltage. In practice, however, the measuring device may not be optimally in tune with the primary resonator. Hence, there is room for improvement of the gyroscope's measuring sensitivity.
The object is therefore to create a measuring device of the aforementioned type with which a high measuring sensitivity is achievable.